Electric power steering systems are increasingly more preferred for motor vehicles of relatively compact design not only for their compactness but also a certain freedom of control they provide. Referring to FIG. 12 showing a typical conventional electric power steering system, this system comprises a steering shaft 2 which is integrally attached to a steering wheel 1, and a pinion 4 of a rack and pinion mechanism which is connected to the lower end of the steering shaft 2 via a connecting shaft 3 having a pair of universal joints 3a and 3b at its either axial end. The rack and pinion mechanism further comprises a rack 8 which can move laterally of the vehicle body and meshes with the pinion 4. The two ends of the rack 8 are connected to knuckle arms 7 of the right and left front wheels 6 via tie rods 5, respectively.
This steering system additionally comprises an electric motor 10 which is coaxially disposed with respect to the rack 8, a ball-nut mechanism 10a for converting the rotational torque of the electric motor 10 to the thrust of the rack 8, a manual steering torque sensor 11 for detecting the manual steering torque Ts applied to the steering wheel 1, a steering angle sensor 12 which detects the angular position of the steering shaft 2 or a steering angle .theta.s, and an assist torque control unit 13 for controlling the output of the electric motor 10 according to the detected values of Ts and .theta.s.
Referring to FIG. 13, the assist torque control unit 13 comprises a target assist torque generating unit 14 which determines an assist torque T0 which the electric motor 10 should produce, and a motor drive unit 15 which controls the electric motor 10 according to this target assist torque T0.
According to such a conventional power steering system, when the vehicle is traveling over a slippery (low-.mu.) road, for instance due to a snow accumulation or freezing, because of a sharp reduction in the steering reaction from the road surface, the assist torque tends to be excessive. Therefore, the force required to turn the steering wheel is so reduced that the vehicle operator is required to be cautious not to excessively steer the vehicle. This is known to cause a substantial stress to the vehicle operator.
To avoid the problems which may arise because of the tendency to excessively steer the vehicle on a low-.mu. road surface, it is conceivable to control the magnitude of the assist torque according to the frictional coefficient of the road surface. However, to accomplish this goal, a separate sensor for detecting the frictional coefficient between the road surface and the tire becomes necessary. According to one known method for detecting the road frictional coefficient, the frictional coefficient is computed from a difference between the rotational speeds of the front and rear wheels. However, this method requires speed sensors for both front and rear wheels, and therefore tends to be complex and expensive. Furthermore, the frictional coefficient of the road surface can be detected only when one of the wheels is slipping due to acceleration or deceleration.
Reference should be made to copending U.S. Pat. No. 5,729,107 issued Mar. 17, 1998, based on Japanese patent application No. 7-245408 filed Aug. 29, 1995, which discloses a device for controlling an electric power steering system according to an estimated road frictional coefficient, and the contents of that patent are incorporated herein by reference.